(Supported in part by CNRS grants to M. Bornens) This is part of an ongoing study of the structure of the human centrosome. Because centrosomes are extremely small (0.25 by 0.5 (m) and complex, the only way their morphology can be studied is with EM. Normally, centrosomes are examined in the EM after embedding and observation in conventional plastic sections. The resolution obtainable is somewhat limited using this technique. Our lab has isolated human centrosomes, and we have been studying them after negative staining. We have obtained high-resolution images showing the periodicity of the centriolar structure. To study the 3-D structure, we needed the IVEM automated tomography system because of the thickness of the whole-mount preparations and the dose-sensitivity of the negative-stain preparations. We also contemplate frozen-hydrated work in the future. On her visit Dr. Ruiz recorded six double-tilt series and one single-tilt tomographic series. A test reconstruction was made by Dr. Rath. This project will continue with the assistance of Dr. Radermacher.